It has been recognized that due to environmental concerns and newly enacted rules and regulations, saleable petroleum products must meet lower and lower limits on contaminates, such as sulfur and nitrogen. New regulations require essentially complete removal of sulfur from liquid hydrocarbons that are used in transportation fuels, such as gasoline and diesel. For example, ultra low sulfur diesel (ULSD) requirements are typically less than about 10 wppm sulfur.
Hydroprocessing is a process that contacts a selected feedstock and hydrogen-containing gas with suitable catalyst(s) in a reaction vessel under conditions of elevated temperature and pressure. The hydrogen is conventionally a separate phase in a three-phase system (gas/liquid/solid catalyst). Such hydroprocessing is commonly undertaken in a trickle-bed reactor where the continuous phase is gaseous and not liquid. The continuous gas phase is far in excess of stoichiometry requiring gas recovery, clean-up, compression and recycle back to the hydroprocessing reaction vessel.
Continuous liquid phase hydroprocessing with a liquid hydrocarbon stream and solid catalyst has been proposed to convert certain hydrocarbon streams into more valuable hydrocarbon streams with less hydrogen requirements. Continuous liquid phase hydroprocessing can operate without a gas recycle stream because the hydrogen requirements can be just over stoichiometric.
Hydrotreating is a type of hydroprocessing primarily active for the removal of heteroatoms, such as sulfur and nitrogen, and saturation of compounds in the hydrocarbon feedstock. Hydrotreating can typically increase the cetane number of a hydrocarbonaceous feed and prepare the feed for further hydroprocessing.
Fractionation columns are necessary to separate hydroprocessed effluents into separate products as distinguished by boiling point ranges. Naphtha, diesel and heavier products are typically recovered from hydroprocessed effluents.
There is a continuing need, therefore, for improved and cost effective methods to produce hydrocarbon streams that meet increasingly stringent product requirements. In particular, there is a need to provide ULSD in a cost effective and efficient manner without overtreating the heavier portions of the product streams.